This invention relates to a method and apparatus for forming an image on an optical recording disc such as an optical disc master.
Optical recording discs such as compact discs have come into widespread use. Typically, a pattern of carefully dimensioned and spaced quarter wavelength pits is formed in a reflective surface in order to encode digital information.
One form of compact disc, known as a CD single, provides reduced playing time, generally up to twenty minutes. When a CD single is formed as a conventionally sized compact disc, this leaves a considerable portion of the area of the disc not devoted to recording audio information.
Recently, this unrecorded area has been used to display graphical images. One approach is to prepare a master by exposing photoresist in a regular pattern of spaced dots throughout the unrecorded area and then to expose selected regions of the unrecorded area through a mask shaped in the desired image thereby removing the regular pattern of exposed photoresist in the selected regions. When the photoresist is developed in the usual way, a graphical image corresponding to the exposed image is left on the master.
This approach brings with it disadvantages because a separate image exposure step is required, which must be done subsequent to conventional mastering.
Conventional compact discs also include human readable text recorded in a diffraction pattern image near the inside radius of the recorded surface. In the past, these letters have been formed during the mastering process with a character generator which modulates the writing beam during the mastering process to form the diffraction image pattern. Such character generators are limited in the number of characters and the complexity of the image that can be produced. For this reason, the character generator approach is not well suited to provide a pleasing graphical display on the relatively large unrecorded area of a CD single.
It is accordingly a principal object of this invention to provide an improved method and apparatus that allows a user to choose a graphical image with great flexibility and ease and to record the graphical image directly on an optical recording disc such as a CD master during the mastering process. In this way, the need for exposure steps following the conventional mastering process is completely eliminated, and excellent graphical images are provided simply and easily.